For an electrical apparatus that is connected to an alternating-current power supply line for use, it is necessary to conduct an insulation resistance test and a withstand voltage test as quality and safety verification tests complying with safety standards such as IEC60598, UL1598, JIS C8105-1 “Luminaires-Part 1: General requirements for safety”. For example, the insulation resistance test and the withstand voltage test are needed to be conducted for an LED lighting apparatus connected to a commercial alternating-current power supply.
For example, Patent Documents 1 and 2 are cited as prior art documents concerning an LED lighting apparatus in which a plurality of light-emitting diodes are provided.    Patent Document 1: JP-A-2009-301952    Patent Document 2: JP-A-2010-80844
FIG. 1 is a schematic side view showing an LED lighting apparatus 100 and a light source board 10 provided therein. The LED lighting apparatus 100 includes the light source board 10 provided with a plurality of light-emitting diodes D (D1, D2, D3) connected in series, a housing 30 to which the light source board 10 is attached, a power supply unit 40 fixed inside the housing 30, and reflector plates 50 reflecting the light of the light-emitting diodes D. A direct-current voltage generated by the power supply unit 40 connected to an external alternating-current power supply is applied across the lightemitting diodes D, such that the light of the light-emitting diodes D is radiated upward in the drawing.
The light source board 10 is formed of the light-emitting diodes D mounted on a radiation substrate 20 in order to improve thermal dissipation of the light-emitting diodes D. A specific example of the radiation substrate 20 is an MCPCB (Metal Core Printed Circuit Board). The MCPCB is a printed board formed of a conductive layer 12 in which copper connections 12a-12f are formed, a thermal conductive insulating layer 11, and a metal plate 14 which are laid one on top of another in this order. For example, an anode-side electrode of the light-emitting diode D1 is soldered to the copper connection 12a, and a cathode-side electrode is soldered to the copper connection 12b. The same goes for the other light-emitting diodes D2 and D3. The housing 30 also functions as a heat sink to improve thermal dissipation of the light-emitting diodes D and the power supply unit 40.
The above-mentioned insulation resistance test, withstand voltage test, etc. are sometimes conducted on the light source board 10 alone or conducted on the LED lighting apparatus 100 as a whole in a state in which the light source board 10 is attached to the housing 30 as shown in FIG. 1. By applying a high-voltage alternating-current voltage between the copper connections of the radiation substrate 20 and the housing 30 (that is, a case ground) by a high-voltage AC power supply 200, it is possible to check electrical insulation performance of the radiation substrate 20 of the light source board 10.
FIG. 2 is a diagram showing the configuration of the light source board 10 and a method for conducting a withstand voltage test. As in FIG. 1, the light source board 10 is formed of an LED string in which a plurality of light-emitting diodes D (D1-D6) are connected in series, and the LED string is mounted on the radiation substrate 20. Between each of copper connections N (N1-N5) which is located between the adjacent light-emitting diodes and a case ground, a corresponding one of parasitic capacitances CP (CP1-CP5) exists. The case ground corresponds to the housing 30 or the metal plate 14 shown in FIG. 1, the copper connections N1-N5 correspond to the copper connections 12b-12e shown in FIG. 1, and each copper connection M1 (M2) at one of the ends of the LED string corresponds to the respective copper connection 12a (12f) shown in FIG. 1.
When a withstand voltage test is conducted, both an end M3 of a power supply line 13a connected in series to the cathode-side end of the LED string and an end M4 of a power supply line 13b connected in series to the anode-side end of the LED string are connected to a power supply line 201 connected to the positive-electrode side of the high-voltage AC power supply 200. A power supply line 202 connected to the negative-electrode side of the high-voltage AC power supply 200 is connected to the housing 30 or the metal plate 14. That is, the high-voltage AC power supply 200 and the housing 30 (or the metal plate 14) are connected to a common electrical ground.
When a high-voltage alternating-current voltage is output from the high-voltage AC power supply 200 in this connection state, although insulation is originally provided between the power supply lines 13 (13a and 13b) and the case ground at the design stage, current paths passing through the parasitic capacitances CP are generated between the power supply lines 13 (13a and 13b) and the case ground.
FIG. 3 shows the results of simulation of a voltage between the terminals (a voltage between the electrodes), the voltage generated across the ends of each light-emitting diode, when an alternating-current voltage of 1000 V/50 Hz is applied between the power supply lines 13 (13a, 13b) and the case ground in the light source board 10. As a result, a reverse voltage of up to about −8 V is observed as a voltage between the electrodes of the light-emitting diode. The highest reverse voltage was observed at the light-emitting diodes D1 and D6, which are located at the ends of the LED string.
It is considered that the observed reverse voltage of the light-emitting diode D1 at one end of the LED string is particularly high because, when the potential of the power supply line 13 (13a, 13b) is higher than that of the case ground as a result of the application of a high-voltage alternating-current voltage, a current flows through a path of the parasitic capacitance CP1, whose impedance is lower than that of the path passing through the inner light-emitting diodes of the LED string. It is similarly considered that the observed reverse voltage of the light-emitting diode D6 at the other end of the LED string is particularly high because, when the potential of the power supply line 13 (13a, 13b) is lower than the case ground as a result of the application of a high-voltage alternating-current voltage, a current flows through the light-emitting diode D6 by passing through a path of the parasitic capacitance CP5 with a low impedance.
As a protection circuit protecting the light-emitting diodes from electrical stress caused by such an excessively high reverse voltage, bypass capacitors 6 arranged as shown in FIG. 8 are considered. FIG. 8 is a configuration diagram of an LED circuit 7 disclosed in Patent Document 2 mentioned above. In the LED circuit 7, a plurality of bypass capacitors 6 whose combined capacitance is greater than the floating capacitance Cs between light-emitting diodes 5 and an earth E are each connected in parallel to a corresponding one of the plurality of light-emitting diodes 5 connected in series.
However, in order to protect the light-emitting diodes from the above-described excessively high reverse voltage by using the protection circuit of FIG. 8, the capacitance of each bypass capacitor 6 has to be increased because the impedance between the anode and the cathode of each light-emitting diode 5 has to be lowered as the number of light-emitting diodes 5 connected in series increases. This increases the size of the bypass capacitor 6 and also increases costs.